1. Field of the Invention
The present invention is generally related to communication systems, and more particularly, to a power control method with discontinuous transmission (DTX) frame detection in these communication systems.
2. Related Art
In a Code Division Multiple Access (CDMA) system, a power control mechanism is typically used to minimize power consumption and interference while maintaining a desired level of performance. Conventionally, this power control mechanism is implemented with two power control loops. The first power control loop (often referred to as an “inner” power control loop, or “inner loop”) adjusts the transmit power to each mobile station such that the signal quality of the transmission received at the mobile station receiver (e.g., as measured by a signal-to-noise ratio) is maintained at a particular target signal-to-noise ratio, or target Eb/N0. The target Eb/N0, where Eb is the energy per information bit, and N0 is the power spectral density of the interference seen by the receiver, is often referred to as a power control set point, or threshold. The second power control loop (often referred to as an “outer” power control loop, or “outer loop”) adjusts the threshold such that the desired level of performance, e.g., as measured by a particular target block error rate (BLER), frame error rate (FER), or bit error rate (BER) for example, is maintained.
For example, for link (e.g., forward link or reverse link) power control, the inner loop compares the threshold to a measured Eb/N0 of the received signal. This is periodically measured in a 1.25 ms interval, for example. If measured Eb/N0 is smaller than the threshold, the receiver requests an increase in power on the link. In other words, there are too many decoding errors when the receiver is decoding frames of a received transmission, such that the FER is outside an acceptable range (i.e., too high). If measured Eb/N0 is larger than the threshold, the receiver requests a decrease in power on the link, i.e., the decoded transmission contain no errors, thus the system may be too efficient (FER is below the acceptable range) and transmit power may be being wasted.
The outer loop surrounds the inner loop and operates at a much lower rate than the inner loop, such as at 20 ms intervals, for example. The outer loop maintains the quality of service of the link. If there was no outer loop, the system would operate at a fixed threshold. But, a fixed threshold may not be adequate for changes in channel and/or environmental conditions, such as changes in channel conditions, transmit power, system load, speed of mobile, rate of channel, decoding of channel, etc. The threshold needs to adaptive to changing channel/environmental conditions. The outer loop looks at quality of the link, and if quality is too poor, the outer loop will increase the threshold accordingly. If link quality is too good, (i.e., an FER less than a target FER of about 1% voice transmissions, higher for data transmissions), the outer loop readjusts the threshold so as not to unduly waste system resources.
Outer loop power control may be adversely affected if the wireless communication system permits discontinuous transmission. Discontinuous transmission, or DTX, is a mode of operation in which a base station or mobile station switches its transmitter on and off autonomously when there is no data to send, so as to avoid releasing the channel, which can be costly in terms of resources, processing inefficiencies, etc. Release of channel destroys data throughput, causing an increase in system down time. Data transmission in the DTX mode reduces transmission power and increases the entire system capacity due to the decrease of interference within the system. DTX mode is an efficient way to use base station/mobile station resources. A system in DTX mode however, exhibits a problem when a receiver does not know whether frames have been transmitted or not, because the transmitter in the DTX mode transmits frames unannounced (i.e., autonomously). This makes it difficult for a receiver (such as a base station, for example) to perform power control.
Frames which are transmitted as DTX frames may have similar Eb/N0 to frames that are transmitted, but which contain errors. Frames that are transmitted and received with errors are referred to as “erasures”. Accordingly, it may be difficult for the receiver to discriminate between frames transmitted with genuine errors and received with errors (erasures) and frames that have not been transmitted with data, i.e., an empty data frame transmitted with zero gain, for example—the DTX frame.
Current DTX detection procedures at the receiver have attempted to distinguish erasures from DTX frames. Current DTX detectors discriminate based on detection of energy (Eb/N0) only. This has proven difficult, especially if signal strength is low. At a low Eb/N0, DTX detection can be unreliable because a DTX frame may be mistaken as an erasure, and vice versa. These cases of mistaken identity may prevent the outer loop to effectively track changes in link due to changed channel/environmental conditions. Thus, the outer loop at the receiver cannot determine whether to maintain or change the threshold. Since the outer loop sets the threshold for the inner loop to meet, inner loop power control is also affected. In other words, if a DTX detector cannot accurately discriminate between erasures and DTX, power control may be inaccurate, potentially wasting power and/or system resources, causing unnecessary retransmissions and/or an inadvertent release of the channel, unnecessarily increasing system down time. This may translate to loss of link performance and system capacity. Accordingly, there exists a need to accurately detect DTX frames.